1. Field of the Invention
This invention relates to a resist composition to be used for a micropatterning technique (or a processing for forming a fine pattern) in the manufacture of a semiconductor device.
2. Description of the Related Art
A micropatterning technique through a photolithography is widely utilized in the field of electronic devices of various kinds such as a semiconductor integrated circuit. This technique is performed as follows. Namely, a photoresist film is formed through a spin coating method onto a semiconductor substrate such as a silicon wafer, the photoresist film thus formed is then exposed to light through a prescribed mask pattern, and then the photoresist film is subjected to various processes such as development and washing with water, thereby forming a resist pattern. This resist pattern is utilized as an anti-etching mask in an etching process wherein exposed portions are etched to form a pattern of fine line or window thereby obtaining a desired pattern.
In the processing of this pattern exposure, a reduced projection type exposure apparatus of a step-and-repeat system, which is generally called as a stepper is widely used. In this case, the g-line (436 nm in wavelength), h-line (405 nm in wavelength) or i-line (365 nm in wavelength) of a mercury vapor lamp, or an excimer laser, such as KrF (248 nm in wavelength), ArF (193 nm in wavelength) or F.sub.2 (157 nm in wavelength) is used as a light source. The shorter the wavelength of light used is, the finer is the pattern that will be produced. Accordingly, it is advantageous to employ a deep UV such as an excimer laser. Further, if an electron ray or an X-ray, which are still shorter in wavelength is employed, a still finer pattern will be obtained.
However, since the conventional resist is large in absorbency to the deep UV, it is impossible in the case of the conventional resist to allow the exposure light to pass into the full depth of the resist film. Accordingly, the sectional shape of pattern to be formed with the conventional resist is different in width between the surface and the bottom thereof. Namely, if the resist is of positive type, the width at the surface thereof is smaller than that of the bottom, and if the resist is of negative type, the width at the bottom thereof is smaller than that of the surface. In either cases, it raises a problem of deterioration in effectiveness of the resist as an etching mask.
In order to solve this problem, the use of a chemically amplified resist has been proposed. This chemically amplified resist is made of a photosensitive composition comprising a compound capable of generating a strong acid as it is irradiated with light (photo-acid generator), and a compound capable of decomposing a hydrophobic group by the catalytic action of the strong acid thus generated, thereby transforming it into a hydrophilic compound.
Examples of this chemically amplified resist are a positive type resist comprising a polymer wherein a hydroxyl group of poly(p-hydroxystyrene) is blocked by a butoxycarbonyl group, and an onium salt, which is a photo-acid generator (H. Ito, C. G. Wilson, J. M. J. Frechet, U.S. Pat. No. 4,491,628(1985)); a positive type resist comprising an m-cresol novolak resin, naphthalene-2-carboxylic acid-tert-butylester and a triphenylsulfonium salt (a photo-acid generator) (M. J. O'Brien, J. V. Crlvello, SPIE Vol. 920, Advances in Resist Technology and Processing, p42, (1988)); and a positive type resist comprising 2,2-bis(4-tert-butoxycarbonyloxyphenyl)propane or polyphthalaldehyde and an onium salt (a photo-acid generator) (H. Ito, SPIE Vol. 920, Advances in Resist Technology and Processing, p33, (1988)).
In these chemically amplified resists, since the acid generated from the light exposure functions as a catalyst, the chemical transformation within a resist can be effectively brought about even if the amount of the photo-acid generator is little. As a result, a chemical reaction can be sufficiently proceeded even in the innermost portion of the resist film where only a little amount of light could be reached. As a result, a resist pattern having a side surface perpendicular to the surface of a substrate can be obtained.
However, since the chemically amplified resists are highly sensitive, it is vulnerable to airborne basic substances and other minority components in the atmosphere. This vulnerability of the resist is reported for example by S. A. MacDonald, et. al., SPIE vol. 1466, Advance in Resist Technology and Processing P2, (1991). For example, dimethylaniline in the atmosphere inactivates the acid generated on the surface of the resist film. As a result, so-called sparingly soluble layer which is very slow in solubility rate to a developing solution is formed on the surface of the resist film. This sparingly soluble layer is left remained in the shape of "T-top" on the surface of the resist pattern after the development step. On the other hand, the sectional shape of the resist pattern to be obtained will be such that the width of opening on the substrate side becomes wider than that on the surface thereof, i.e. the angle between the side wall of the pattern and the surface of the substrate is more likely to become smaller than the ideal angle of 90.degree. so that it would be impossible to form a pattern of rectangular shape in cross section.
Due to the presence of this sparingly soluble layer, the resolution of a resist is lowered, and at the same time the sensitivity thereof becomes unstable. Further, such a shape of pattern to be formed under the presence of this sparingly soluble layer is considered to give a bad influence to the etching accuracy of the active regions of a semiconductor substrate.
In order to avoid the formation of the sparingly soluble layer, a method of forming a protective layer on the chemically amplified resist film to block the influence of airborne amines in the atmosphere has been proposed (for example, Japanese Patent Unexamined Disclosure Sho-63-287950, and Japanese Patent Unexamined Disclosure Hei-4-2040848). However, this method of forming a protective layer raises another problem that it requires a new coating apparatus, and it badly affects the workability due to the increase in number of processing step.
On the other hand, a chemically amplified resist containing a little amount of basic substance for improving the property of resist is proposed (Japanese Patent Unexamined Disclosure Sho-63-149638, Japanese Patent Unexamined Disclosure Sho-63-149639 and Japanese Patent Unexamined Disclosure Sho-63-149640). For example, Japanese Patent Unexamined Disclosure Sho-63-149640 describes a method of inhibiting change in sensitivity after the exposure by incorporating an amine compound into a resist. Further, Japanese Patent Unexamined Disclosure Hei-5-127369 describes a method of improving the resolution by adding a substance which functions as a base against the acid resulting from the irradiation, such as an aniline-based derivative, an imidazole-based derivative, a pyridine-based derivative, or ammonia-based derivative, to a resist.
However, even if the conventional chemically amplified resists containing a basic substance are employed, it is still insufficient to prevent the formation of the T-top shape, or to prevent the phenomenon that the width of cross-sectional shape of resist pattern becomes larger on the substrate side than on the surface side. Therefore, it is still desired to develop a method of forming a resist pattern which is more rectangular in a cross-section.
Further, many of photo-acid generators in a chemically amplified resist composition react with the above mentioned basic compounds as these basic compounds are mixed into the resist thereby losing its capability of generating an acid. Even if the basic compound is weak in basicity, the reaction between the photo-acid generator and the basic compound proceeds slowly depending on the magnitude of the basicity. Because of this, it is difficult to obtain a resist which is tapable of maintaining a stabilized property for a long period of time.